Blender Python: Convert Enum Flags into String?
It is sometime later after posting this question, and I have learned a lot about how Blender properties work. Here are the nuts and bolts that should answer my older question. Below, instance represents a scene element or property group, and Prop represents any property defined for the instance.


All Blender properties are stored in memory as two different value types - complex and basic. Some properties (like int) are the same type, but are still stored in two locations. Some others (like enum flags) are stored as very different types in each location.
Using syntax instance.Prop (same as getattr(instance,"Prop") or setattr(instance,"Prop",*)) reads and/or writes the property using the complex value types.
Using syntax instance["Prop"] reads and/or writes the property using basic value types.
Assigning either version of the two property value types will instantly and automatically update the other version to the same value of its respective type. You can literally assign a set of strings to the complex version of an enum flags property and, on the very next line, read the basic version of it to convert complex -> basic. Or do the opposite to convert the other way.
Be careful when reading the basic version of any property (syntax instance["Prop"]). It will not be generated for a specific instance (eg, property group) until the property is assigned for the first time (either type version). You can check to see if the basic version of a property has been generated by using if "Prop" in instance: or if "Prop" in instance.keys():. You can also use instance.get("Prop",DEFAULT_BASIC_VALUE) to safely request the basic version without knowing if it has been assigned yet.
The basic version of an enum flags property (instance["MyFlags"]) is stored as an int, which represents all of the binary flag values ORed together. The complex version of an enum flags property (instance.MyFlags) is represented as a set of strings, where each string represents the bit or mask you assigned to it.
To convert an enum flags property into a string, you can literally just convert the complex value into a string: str( instance.MyFlags )